Known methods for surface cleaning of physical objects include wet cleaning (such as, brushing with detergent and water and drying, and ultrasonic solvent cleaning) and dry cleaning (such as, laser ablation, carbon dioxide composite cleaning, and plasma cleaning). Plasma cleaning has the advantages of being a fast one-stop dry process, with compatibility or both offline and in-line cleaning.
Conventional plasma cleaning however induces damage to workpieces; this a concern particularly when the object for cleaning is sensitive to damage such as semiconductor wafers, integrated circuit components and living tissues. The damage on such sensitive objects arises due to bombardment of ions and electrons in the plasma and undesirable etching on the surface of the objects. Undesirably, the temperature of the plasma cleaning process is also high, typically being over 100 degC. thus making cleaning of plastic objects impossible due to their low melting temperatures. When a vacuum chamber is involved, it adds to the cost of a large machine footprint and capital expense. In addition, the feedstocks often required for plasma cleaning are toxic and/or flammable. Atmospheric pressure chambers are known in the art but low temperature, high plasma cleaning rates and large area plasma cleaning are desirable features for commercial applications. Obtaining all these desirable features in the same apparatus have remained a challenge.
U.S. Pat. No. 7,754,994 discloses a method for generating atmospheric gas discharge plasma in a gas whirlpool cavity. The plasma is then sprayed out in a gas flow to clean objects one after another in a continuous manner. The process is cheap and simple. U.S. Pat. No. 8,471,171 discloses a micro-hollow cathode discharge assembly that is capable of generating a low temperature and atmospheric pressure plasma micro jet that enables the application of the assembly to low temperature treatments including treatment of living tissues. U.S. Pat. No. 6,906,280 discloses fast pulse non-thermal plasma reactor where the pulses create non-thermal plasma discharge within a discharge cell. Accordingly, the non-thermal plasma discharge can be used to remove pollutants from gases or break the gases into smaller molecules so that they can be more efficiently combusted. U.S. Pat. No. 6,329,297 discloses dilute remote plasma cleaning process, and apparatus for enhancing its etching rate and uniformity. The plasma remotely formed is diluted before flowing into a processing chamber to clean or etch an object or the interior of the chamber itself. U.S. Pat. No. 6,835,678 discloses a system and method for remote plasma where activated species from a remote plasma generator is delivered into a processing chamber.
Though plasma cleaning is preferred over many other cleaning methods, improvements are desirable to lower the process temperature, to perform the cleaning process at atmospheric pressure, to reduce ion and electron bombardment damage, fast processing for increased throughput and large surface cleaning, and to perform the cleaning process without the use of toxic or flammable feedstock gases. Also desired is plasma cleaning in air or in a liquid medium.